1. Field of the Invention:
The present invention relates to a power unit structure for a hybrid vehicle, having an engine and a motor as drive power sources.
2. Description of Background Art:
In the background art, a known hybrid vehicle has power transmission means for transmitting power from an engine to a drive wheel housed inside a transmission case. The power transmission means combines and transmits power from the engine and power from a motor to a drive shaft (see, for example, Japanese patent laid-open No. Hei. 8-175473). The hybrid vehicle disclosed in the above document has a generator connected directly or though a reduction gear to a driven shaft of the power transmission means.
However, when a requested shaft torque is supplied to the drive wheel, with the above-described configuration having the motor connected via a reduction gear to the driven shaft of the power transmission means, a need arises to provide an additional reduction gear independent from the first reduction gear. In addition, with the above-described configuration where the motor is directly connected to the driven shaft of the power transmission means, it is difficult to ensure a sufficient reduction ratio from the motor to the drive wheel. Therefore, there is a problem in that the motor is increased in size.
The hybrid vehicle disclosed in the above patent document also has the drive wheel connected to the drive shaft projecting in a widthwise direction of the vehicle from the unit case, and a generator is connected to an outer side in the widthwise direction of the vehicle. Therefore, the thickness around the drive shaft becomes wide.
To deal with this, it has been considered to house the generator inside the unit case to make efficient use of space. However, the generator of a hybrid vehicle supplies traveling force to the drive shaft. Therefore, a rated output normally becomes large, and the amount of heat generated also becomes large. Therefore, since the width around the drive shaft is narrowed, when the generating motor is housed inside the power unit it is desirable to improve cooling capability of the generating motor.
In the background art, a vehicle provided with, for example, a belt type continuously variable transmission as power transmission means for transmitting power from an engine to a drive wheel is known (see, for example, Japanese patent application No. 2002-181147). The vehicle disclosed in this document is configured with a centrifugal starter clutch connected to a crankshaft of an engine. If the rotational speed of the engine exceeds a specified value, the starter clutch is engaged to rotate the belt type continuously variable transmission. In addition, engine power is transmitted to the drive wheel through the belt type continuously variable transmission. Specifically, until the rotational speed of the engine exceeds a specified value, the belt type continuously variable transmission does not rotate, giving the effect of lowering friction.
However, the vehicle disclosed in the above-described patent document is normally a motorcycle, and not a hybrid vehicle where engine power and motor power are combined and transmitted to the drive wheel side. In a hybrid vehicle, there are cases where a drive wheel is driven with only a motor as a power source. It is desirable to improve energy transmission efficiency in this type of case. Also, in the case where rotational speed of the engine is low, such as when idling, it is desirable to use a rotational force of the crankshaft efficiently, and to generate electricity with the motor functioning as a starter motor and a generator.